Gas extraction

ABSTRACT

An electrochemical cell used to separate gas from a gaseous mixture by reduction of said gas at the cathode and regeneration of said gas at the anode is characterized in that one or more substances formed during the cathodic reduction and/or the anodic regeneration is chemically converted, preferably by catalytic decomposition, to produce further quantities of said gas, the gas formed by both the anodic regeneration and the chemical conversion being recovered as the product. In an especially preferred embodiment a plurality of said cells are used in apparatus for extracting oxygen from the air by using a cathode comprising high surface area graphite powder which reduces oxygen to produce peroxyl ions, each cell being further provided, externally of the cathode compartment, with means for catalytically decomposing the peroxyl ions produced, suitable catalysts being CoFe 2  O 4  or NiCo 2  O 4 .

This is a division of application Ser. No. 137,114, filed as PCTGB79/00060, Apr. 11, 1979, published as WO79/00933, §102(e) date Nov. 5,1979, now U.S. Pat. No. 4,300,987.

This invention relates to the extraction of gases from gaseous mixturesand, more especially, to the extraction of oxygen from ambient air.

Portable oxygen providing equipment is finding increasing applicationsin military field hospitals, in the village clinics of developingcountries, in oxy-acetylene welding and in high altitude fighteraircraft. Besides oxygen cylinders, such equipment comprises two basictypes of oxygen generator; (a) chemical generators, and (b)electrochemical generators, which either electrolyse water or elseextract oxygen from the air by using an oxygen reduction cathode coupledto an oxygen evolution anode.

Chemical generators employ expensive chemicals and, to date, the mostcommon method used to generate oxygen has been the electrolysis ofwater. However, the high overvoltage at the anode necessitates anoperating voltage of about 2.2 volts at 100 mA/cm² and the production ofhydrogen as a by-product, apart from increasing the power consumption,is a potential hazard. Quite recently, therefore, there has beendeveloped a more efficient electrochemical method in which apolytetrafluoroethylene (PTFE) bonded platinum black cathode is used toreduce oxygen in the ambient air to form hydroxyl ions in accordancewith the following equation:

    O.sub.2 +2H.sub.2 O+4e→40H.sup.-

The hydroxyl ions diffuse through a suitable membrane, for example, awoven asbestos or glass fibre mat, to an oxygen evolution anode, usuallya platinum electrode. The working voltage of this cell, taking intoaccount the unavoidable heat losses, is about 1.3 to 1.4 volts at 100mA/cm², which is significantly lower than the corresponding value for anelectrolysis cell. However, the capital and running costs involved arestill relatively high and the use of this type of oxygen extractor hasbeen limited to specialised applications.

The present invention is based on our surprising observation that theefficiency of such electrochemical gas extraction and regenerationmethods can be improved significantly in certain applications byemploying an additional chemical step in which one or more subtancesformed in the electrochemical reaction is itself converted to thedesired gaseous product, preferably by catalytic decomposition.

Accordingly, the present invention provides a method in which anelectrochemical cell is used to separate a gas from a gaseous mixture byreduction of said gas at the cathode and regeneration of said gas at theanode, characterised in that one or more substances formed in thecathodic reduction and/or the anodic regeneration is chemicallyconverted to produce said gas and in that the gas formed by both theanodic regeneration and the chemical conversion is recovered as theproduct. The present invention also provides apparatus for carrying outsuch a method.

In its broad aspect the present invention may be applied to theextraction of any gas from a gaseous mixture, provided that said gas mayselectively be cathodically reduced and anodically regenerated byelectrochemical reactions which produce a substance which can bechemically converted to said gas, preferably by catalytic decomposition.However, it will be appreciated from the foregoing remarks that theinvention is especially applicable to the extraction of oxygen fromambient air and, for convenience, it will now be described in moredetail with reference to such an application.

It is known that the reduction of oxygen on certain electrodes,especially graphite and carbon, is a two electron process obeying theequation

    O.sub.2 +H.sub.2 O+2e→HO.sub.2.sup.-+OH.sup.-

This means that when such electrodes are used in a fuel cell, they arenot so efficient as electrodes which directly reduce oxygen to hydroxylions via a four electron process. It has, therefore, been proposed toadd peroxide-decomposition catalysts to a carbon cathode in a fuel cellso that the peroxyl ion is decomposed to yield oxygen for recycling,thus leading to a higher efficiency. In accordance with the presentinvention; however, the catalytic decomposition of the peroxyl ion isused in an oxygen generator to provide an additional supply of oxygen asproduct.

In an especially preferred form of the present invention, therefore,there is provided an oxygen extraction apparatus comprising at least onecell which is provided with a cathode for the reduction of oxygen by anelectrochemical reaction which produces peroxyl ions and an anode whichwill regenerate oxygen characterised in that said cell is furtherprovided, externally of the cathode compartment, with means for thecatalytic decomposition of hydrogen peroxyl ions.

It will be appreciated that such apparatus will also be provided withmeans for the supply of ambient air at a suitable rate and also withmeans for collecting the oxygen produced by the electrochemical andchemical reactions. In practice the apparatus will typically comprise aplurality of such cells, for example from 10 to 20, and mayadvantageously be built to a modular design for ease of replacement andrepair of the cells. It will, in any case, be appreciated that thedesign of the apparatus will depend upon the particular application and,especially, upon the desired rate of production of oxygen.

Suitable physical constructions for the cell will be apparent to thoseconversant with fuel cell technology. For example it will, in general,be appropriate to provide means such as a porous screen made, forexample, from asbestos or glass to separate the cathode compartment fromthe remainder of the cell in order to prevent back diffusion of oxygenbubbles to the cathode compartment and to prevent diffusion of airbubbles into the product stream. If the cell is compact in nature it maybe necessary to provide means such, for example, as a net made from aplastics or other insulating material, to ensure that there is noelectrical contact between the anode and the catalytic means.

Other constructional features which may prove advantageous or necessaryin certain applications include means for removing carbon dioxide, meansfor removing electrolyte, e.g. potassium hydroxide, entrained in theoxygen product stream and means for controlling the oxygen content ofthe final outlet gas by admixture of the oxygen product stream withexhaust gas from the cathode compartment; this is essential in somemedical applications. It will, in addition, in general be necessary toprovide adequate means for controlling the voltage, current density,temperature and operating pressure of the cell and for monitoring theoverall heat and mass balance of the apparatus.

In this preferred application of the present invention it is necessaryfor the cathode to reduce oxygen by an electrochemical route whichproduces peroxyl ions, usually together with hydroxyl ions (generallyvia the two electron process described above or a near stoichiometricvariation thereof) but which is made from a material which does notpossess significant peroxide decomposition activity. For example,cathode materials such as fuel cell grade platinum black produce peroxylions but, because they have a relatively high peroxide decompositionactivity, will not, in general, be suitable for use in the presentinvention because the peroxide ion will be decomposed in the cathodecompartment and the oxygen so produced cannot be collected with theoxygen produced at the anode.

In general it is preferred to use a high surface area graphite powderwhich has been found to possess a very high activity for oxygenreduction in alkaine solution. A suitable graphite powder having asurface area in the range of 500 to 600 m² /g may be prepared by vacuumgrinding of graphite in a vibrating ball mill; the cathode may beprepared from the graphite powder by bonding with PTFE. However, it willbe appreciated that other materials may be suitable for the cathode incertain applications.

The main criterion for the oxygen evolution anode is that it shouldoperate at a low oxygen overvoltage. Thus certain anodes such asPTFE-bonded platinum black and nickel screens will not, in general, besuitable for use in the present invention. However, amongst suitablematerials there may be mentioned, for example, PTFE-bonded lithiatednickel oxide and, especially, PTFE-bonded nickel cobalt oxide (NiCo₂ O₄)which has a lower redox potential than a platinum black anode.

Both the cathode and anode may, in some cases, advantageously be formedby depositing the active materials on a suitable support, such as anickel screen.

The electrolyte itself is advantageously an aqueous solution of analkali metal hydroxide, e.g. sodium hydroxide, or especially, potassiumhydroxide, but it will be appreciated that other electrolytes may beuseful in certain applications.

The essence of this preferred application of the present invention is toprovide means for catalytically decomposing hydrogen peroxide (or, morecorrectly, the peroxyl ion which it forms in the electrolyte) to producean additional supply of oxygen for collection in the anode compartment.In this way, the effective current required for the oxygen extractionprocess is halved leading to a significant reduction in powerconsumption. In certain instances it may be appropriate to provide suchmeans as a chemical substance in solution or dispersion in theelectrolyte, but, in general, it will be preferred to provide thecatalyst in the form of a solid member or, more especially, absorbed orcoated on a suitable solid support. Amongst suitable materials there maybe mentioned, for example, certain spinel oxides and silver which may becoated or absorbed on a graphite or carbon support. Preferred peroxidedecomposition catalysts are CoFe₂ O₄ and, especially, NiCo₂ O₄. Thesematerials may be brought into suitable form by bonding with PTFE, but,in general, it will be preferred to coat them on a suitable porous orperforated support, e.g. a nickel screen. It has been found that theiractivity is considerably greater in the latter form because thecatalytic reaction can more easily occur on the surface of the screenand because, in addition, the available surface of the screen isentirely composed of the catalyst whereas about 50% of a PTFE-bondedmember is composed of PTFE particles where no catalytic reaction canoccur.

Both CoFE₂ O₄ and NiCo₂ O₄ are relatively cheap materials which can beproduced in particulate form by freeze-drying methods or by thermaldecomposition or double precipitation methods.

In summary, it may be stated that the electrochemical/chemical method ofoxygen extraction according to the present invention providessignificant advantages in cost reduction and in other respects ascompared with the conventional electrochemical oxygen extractors atpresent available. At operating voltages of around 1 volt and at anoperating temperature of 40° C. with 5N KOH as the electrolyte theapproximate power consumption of a cell constructed in accordance withthe present invention is only about 2.7 to 3 kilowatt hours per 1,000liters of oxygen produced whereas conventional extractors require about4.4 kilowatt hours. Rough calculations indicate that if the cells of thepresent invention are used in an efficient manner they will be capableof producing oxygen in a relatively cheap manner and will lead tosignificant savings in cost in applications where oxygen cylinders areat present employed.

The following Examples illustrate the invention. The FIGURE showsdiagrammatically apparatus for extracting gases from gaseous mixtures inaccordance with the invention. A three compartment cell was constructedas shown in the FIGURE. It will be appreciated that the cell shown wasconstructed for experimental evaluation of various aspects of thepresent invention and is not to be considered as typical of a cell to beused commercially.

The cell generally comprises a cathode compartment 1, a catalyticdecomposition compartment 2 containing a catalytic decomposer 15, and ananode compartment 3. The cathode compartment 1 was provided with afloating electrode 4 and, as shown, a Dynamic Hydrogen Referenceelectrode (DHE) 5 was used to monitor the potential working electrode 4,the tip of the Luggin capillary 6 of the DHE electrode 5 beingpositioned about 1 mm below the floating electrode 4. If desired, thecathode compartment 1 may be separated from the remainder of the cell bythe provision of a No. 4 glass frit 7. 5N KOH solution was used as theelectrolyte 8. A catalytic decomposer 15 was provided in compartment 2.The anode compartment 3 was provided with a working anode 9 and with aDHE electrode 10. Air was passed into cathode compartment 1 via inlet 11and the oxygen produced in compartments 2 and 3 was collected by meansof outlets 12 and 13, respectively. As shown, outlets 12 and 13 led to afurther combined outlet 14.

EXAMPLE 1

In this Example a cell was used having two compartments separated by aNo. 4 glass frit and provided with a 8 cm² PTFE-bonded graphite floatingcathode and a 20 cm² nickel screen anode. Current was passed at thefixed densities shown using a Chemical Electronics Potentiostat (TR40-3A). The 5N KOH electrolyte (80 ml) was thermostated at 25° C. and anair pump was used to supply air to the cathode compartment.

After 1 hour, the concentration of H₂ O₂ in the electrolyte was measuredby titration against standard potassium permanganate solution. The selfdecomposition of H₂ O₂ in 5N KOH under the same condition (but withoutpassing current) was measured and found to be 0.525 g/liter of HO₂ ⁻ perhour. The numbers of runs is shown in brackets.

Table 1 shows the amounts of HO₂ ⁻ produced at different currentdensities and confirms that the reduction of oxygen on graphite proceedsvia the two electron process.

The 74% yield of HO₂ ⁻ as compared with theoretical was mainly due todecomposition on the nickel screen anode and losses during pipetting andtitration.

                  TABLE 1                                                         ______________________________________                                                           Experimental                                                                  average gm HO.sub.2.sup.-                                                                   Experimental/                                Current Theoretical                                                                              (in 80 ml.    Theoretical                                  mA      gm HO.sub.2.sup.-                                                                        electrolyte)  %                                            ______________________________________                                        200     0.1230     0.0917        74.55 (2)                                    400     0.2460     0.1866        75.85 (2)                                    600     0.3693     0.2675        72.40 (2)                                    800     0.4924     0.3575        72.6 ± 0.02 (4)                           ______________________________________                                    

EXAMPLE 2

Various electrode combinations were evaluated using the cell shown inthe FIGURE. A Chemical Electronics Potentiometer (TR 40-3A) was used tofeed a 400 mA current to the cell for 30 minutes and a gas burette wasattached to outlet 14 to measure the amount of oxygen produced (exceptthat the glass frit 7 was omitted).

The PTFE-bonded cathodes were prepared in a conventional manner, theNiCo₂ O₄ /nickel screen catalytic decomposers and the PTFE-bonded NiCo₂O₄ /nickel screen anodes were prepared as follows:

(a) NiCo₂ O₄ /nickel screen catalytic decomposers. 100 mesh nickelscreen were dipped into 2M Ni/Co nitrate solution (Ni:Co=1:2) and thescreens were heated in air at 400° C. for 10 hours. The formation ofNiCo₂ O₄ spinel on the surface of the screen was confirmed by X-raypowder diffraction.

(b) PTFE-bonded screen anodes. NiCo₂ O₄ (prepared by freeze drying) wasmixed with GP1-Fluon dispersion (ex ICI). The NiCo₂ O₄ :PTFE ratio was10:3. The mixture was then painted onto a 100 mesh nickel screen anddried at 100° C. for 1 hour.

The results obtained with the various electrode and catalytic decomposercombinations are shown in Table 2.

It will be seen that the best results were obtained when operating inaccordance with the present invention (cells A and B) using aPTFE-bonded graphite/screen cathode and a PTFE-bonded NiCo₂ O₄ /nickelscreen anode with a NiCo₂ O₄ /nickel screen catalytic decomposer.

When no catalytic decomposer was employed (cell C) the power consumptionwas similar to that with a cell using platinum black as the cathode(cell D). Even though, in cell C, the cathodic reduction of oxygenoccurred by a two electron process, the peroxide ions diffusing to theanode were not fully decomposed and when the power was switched offgassing continued slowly for a long time on the PTFE-bonded NiCo₂ O₄anode surface (which is not so active as the thermally decomposed NiCo₂O₄ /nickel screen catalytic decomposer). This suggests that when theanode is functioning electrochemically the oxygen bubbles effectivelyblanket a large part of its surface, making it unavailable for peroxidedecomposition.

When a peroxide decomposer was placed between a platinum cathode and theNiCo₂ O₄ anode (cell E) no improvement was noted. This is not surprisingbecause any HO₂ ⁻ ions produced at the platinum cathode would bedecomposed there, yielding oxygen for further cathodic reduction.

It will be seen that in all cases the power consumption wassignificantly lower than for a conventional water electrolyser (cell F)and that cell B, constructed in accordance with the present invention,gives significantly better results than the previously proposed cell(cell G) using a platinum black anode and cathode.

Finally, it will be noted that in the experimental cells A and B, notall the peroxyl ions produced were decomposed. This is mainly because ofthe large distance between the anode and cathode and significantimprovements in peroxide conversion can be obtained by decreasing theanode/cathode gap.

                                      TABLE 2                                     __________________________________________________________________________    Extraction of oxygen from air (400 mA passed through cell for 30 min)                                      Theoretical                Power                                              From H.sub.2 O.sub.2                                                                  From O.sub.2  Voltage                                                                            consumption           Cell           H.sub.2 O.sub.2                                                                             decomposition                                                                         evolution                                                                           Experimental                                                                          iR free                                                                            KW hr/1000            No.                                                                              Temp.                                                                             Cathode decomposer                                                                          Anode   ml (NTP)                                                                              ml (NTP)                                                                            ml (NTP)                                                                              volt liters of             __________________________________________________________________________                                                            O.sub.2               A  25° C.                                                                     4 cm.sup.2 Teflon                                                                     4 cm.sup.2                                                                          4 cm.sup.2 Teflon                                                                     45.60   45.60 78.03 ± 3.18                                                                       1.34 3.46                         bonded  NiCo.sub.2 O.sub.4                                                                  bonded                                                          graphite                                                                              on    NiCo.sub.2 O.sub.4                                              on nickel                                                                             nickel                                                                              on nickel                                                       screen  screen                                                                              screen                                                                        (O.sub.2 evolving)                                       B  40° C.                                                                     4 cm.sup.2 Teflon                                                                     4 cm.sup.2                                                                          4 cm.sup.2 Teflon                                                                     45.60   45.60 77.04 ± 3.02                                                                       1.04 2.69                         bonded  NiCo.sub.2 O.sub.4                                                                  bonded                                                          graphite                                                                              on    NiCo.sub.2 O.sub.4                                              on nickel                                                                             nickel                                                                              on nickel                                                       screen  screen                                                                              screen                                                                        (O.sub.2 evolving)                                       C  25° C.                                                                     4 cm.sup.2 Teflon                                                                     none  4 cm.sup.2 Teflon                                                                     45.60   45.60 49.82 ± 1.11                                                                       1.34 5.41                         bonded        bonded                                                          graphite      NiCo.sub.2 O.sub.4                                              on nickel     on nickel                                                       screen        screen                                                                        (O.sub.2 evolving)                                       D  25° C.                                                                     4 cm.sup.2 Teflon                                                                     none  4 cm.sup.2 Teflon                                                                     N/A     45.60 46.96 ± 0.68                                                                       1.26 5.36                         bonded        bonded                                                          Platinum      NiCo.sub.2 O.sub.4                                              black on      on nickel                                                       Pt screen     screen                                                          (O.sub.2 reduction)                                                                         (O.sub.2 evolving)                                       E  25° C.                                                                     4 cm.sup.2 Teflon                                                                     4 cm.sup.2                                                                          4 cm.sup.2 Teflon                                                                     N/A     45.60 47.77   1.26 5.26                         bonded  NiCo.sub.2 O.sub.4                                                                  bonded                                                          Platinum                                                                              on    NiCo.sub.2 O.sub.4                                              black on                                                                              nickel                                                                              on nickel                                                       Pt screen                                                                             screen                                                                              screen                                                          (O.sub.2 reduction)                                                                         (O.sub. 2 evolving)                                      F  25° C.                                                                     4 cm.sup.2 Teflon                                                                     none  4 cm.sup.2 Teflon                                                                     N/A     45.60 45.80   2.09 9.12                         Pt            bonded                                                          black on      NiCo.sub.2 O.sub.4                                              Pt screen     on nickel                                                       (H.sub.2 evolving)                                                                          screen                                                                        (O.sub.2 evolving)                                       G  40° C.                                                                     Teflon  none  Teflon  N/A     45.60 45.60   1.0  4.38                         bonded        bonded                                                          Pt electrode  Pt electrode                                                    (O.sub.2 reduction)                                                                         (O.sub.2 evolving)                                       __________________________________________________________________________

EXAMPLE 3

The catalytic activity of NiCo₂ O₄ and CoFe₂ O₄ catalytic decomposerswas measured by a conventional gasometric technique.

In all cases the reaction rate constant was found to be independent ofthe initial H₂ O₂ concentration but directly proportional to thecatalyst mass, indicating first order Kinetics.

Experiments were conducted at 25° C., 30° C., 35° C. and 40° C. Byplotting the first order reaction rate constant against the reciprocalof the temperature (degree kelvin) the activation energy for thedecomposition of H₂ O₂ over freeze dried NiCo₂ O₄ (surface area 69.7 m²/g) was calculated as 10.98 Kcal/mole, similar to the value of 10.60Kcal/mole obtained for CoFe₂ O₄ (prepared by coprecipitation of cobaltand iron hydroxides followed by dehydroxylation at 100° C.) having asurface area of 120 m² /g.

Taking differences in surface area into account this shows the activityof NiCo₂ O₄ to be approximately equivalent to that of CoFe₂ O₄. However,NiCo₂ O₄ is easier to prepare and apply to a screen and is thuscurrently preferred for use in the present invention.

It will be apparent from the above description that the presentinvention is especialy advantageous in that it provides an oxygengenerator which is highly efficient, which has a lower power consumptionthan hitherto proposed cells and which can be constructed fromrelatively cheap materials. It will be understood, however, that theelectrochemical/chemical process described is not limited to such anapplication and other applications, modifictions and variations fallingwithin the scope of the present invention will be apparent to thoseskilled in the art.

We claim:
 1. Oxygen extraction apparatus comprising at least one cellincluding a cathode for the reduction of oxygen by an electrochemicalreaction thereby to produce peroxy ions, an anode adapted to regenerateoxygen, and said cell including, spaced apart from the cathode, means,other than the anode, for the catalytic decomposition of said peroxylions.
 2. Oxygen extraction apparatus comprising at least one cellincluding a cathode for the reduction of oxygen by an electrochemicalreaction thereby to produce peroxyl ions, an anode adapted to regenerateoxygen, means for effecting catalytic decomposition of said peroxyl ionsto yield oxygen, said catalytic decomposition means being spaced apartfrom the cathode and not being electrically connected to either thecathode or anode.
 3. Oxygen extraction apparatus as claimed in eitherclaim 1 or 2 which comprises from 10 to 20 of said cells.
 4. Oxygenextraction apparatus as claimed in either claim 1 or 2 wherein saidcathode comprises a high surface area graphite powder.
 5. Oxygenextraction apparatus as claimed in claim 4, wherein the powder has asurface area in the range of from 500 to 600 m² /g.
 6. Oxygen extractionapparatus as claimed in claim 4, wherein the cathode is made from saidgraphite powder by bonding with polytetrafluoroethylene.
 7. Oxygenextraction apparatus as claimed in either claim 1 or 2 wherein the meansfor the catalytic decomposition of peroxyl ions comprises CoFe₂ O₄ orNiCo₂ O₄.
 8. Oxygen extraction apparatus as claimed in claim 7, whereinthe CoFe₂ O₄ or NiCo₂ O₄ is coated on a porous or perforated support. 9.Oxygen extraction apparatus as claimed in claim 8, wherein said supportis a nickel screen.
 10. Oxygen extraction apparatus as claimed in claim7, wherein the CoFe₂ O₄ or NiCo₂ O₄ is bonded withpolyetrafluoroethylene.
 11. Oxygen extraction apparatus as claimed ineither claim 1 or 2 wherein the means for the catalytic decomposition ofperoxyl ions comprises a spinel oxide or silver.
 12. Oxygen extractionapparatus as claimed in claim 11, wherein the spinel oxide or silver iscoated or absorbed on a graphite or carbon support.
 13. Oxygenextraction apparatus as claimed in either claim 1 or 2 which comprisesmeans for supplying ambient air to the cell at a suitable rate. 14.Oxygen extraction apparatus as claimed in either claim 1 or 2 whichcomprises means for collecting the oxygen produced by theelectrochemical and catalytic reactions.
 15. Oxygen extraction apparatusas claimed in claim 14, which comprises means for removing electrolyteentrained in the oxygen product stream.
 16. Oxygen extraction apparatusas claimed in claim 14, which is provided with means for controlling theoxygen content of the final outlet gas by incorporating exhaust gas fromthe cathode compartment with the oxygen product stream.
 17. Oxygenextraction apparatus as claimed in either claim 1 or 2 wherein there isa porous screen between the cathode and the remainder of the cell. 18.Oxygen extraction apparatus as claimed in claim 17, wherein said porousscreen is made from asbestos or glass.